Falls and mobility disorders are common, dangerous, and costly conditions among older people. Their causes are multifactorial, including impairments in vision, gait, balance, muscle strength and cognition. Loss of peripheral somatosensory function, which is common in aging, diabetes, and other causes of peripheral neuropathy, is also a risk factor for falls. Until recently, there were no proven methods to improve somatosensory function in humans.
Several non-linear biological systems, ranging from ion channels to sensory neurons, used the presence of a particular sub-threshold level of noise to enhance the detection of a weak signal. This phenomenon is known as stochastic resonance (SR), based on which subsensory vibratory noise has been applied to the feet for improving balance in healthy young and elderly subjects, and patients with diabetic neuropathy and stroke. Previous studies have suggested that SR is a potentially viable technology for improving balance and gait if it can be delivered via a shoe insole.
However, the previous studies were problematic because the vibrating tactor required a large energy source that could not be embedded into a shoe. Another problem of the previous studies was that a single baseline sensory threshold was determined through extensive testing, the amplitude of vibration having been set at 90% of this threshold.
Yet another problem of previous studies and methods was that actuator placement and insole construction focused on widely distributing vibration by placing multiple, spatially separated actuators across the insole, and on fabricating the insole from vibration propagating materials for maximizing the distribution of sensory enhancement stimulation throughout the foot surface.
Previously, it was thought that a wider distribution of stimulation was preferable for balance and gait improvement based on the greater stimulation of the field of mechanoreceptors found broadly distributed throughout the foot. As such, previous actuator positions focused on those high density regions. Furthermore, it was also previously described that rigid actuators can be placed in the arch for the purpose of isolating them from known footwear pressure points and bending planes. However, these previous placements were thought to require the use of vibration propagating structures to deliver stimulation from the arch to the areas rich in sensory mechanoreceptors.
The previous wide distribution of stimulation and placements are problematic for at least the following reasons. First, the forefoot and heel regions encounter drastically different pressures throughout the gait cycle. These pressure variations result in a constantly changing mechanical coupling between the vibration sources and the surrounding materials which leads to large changes in applied vibration levels during the gait cycle. Second, the use of vibration propagating structures leads to constructive and destructive interference of vibration patterns. This interference causes peaks and valleys across the insole resulting in difficulty setting the mechanical threshold and therapeutic vibration levels required for this therapy to be effective. In addition, vibration propagating structures are typically rigid, which makes them difficult to incorporate into insoles. Placing these materials in contact with both the skin and actuators proved to be uncomfortable. Third, the use of numerous spatially distributed actuators leads to the therapeutic level being set based on whichever region of the foot is most sensitive. Because all of the actuators are driven by the same driving signal, this can result in the stimulation being set too low.
Therefore, there is a continuing need for solving the above and other problems.